1. Field of the Invention
The invention relates to an absolute length measuring system or position measuring system having a preferably magnetically coded measuring gauge that has an incremental track and at least one absolute track.
2. Description of the Related Art
Incremental Measuring Systems
Magnetic strip length measuring systems have become known in the field of measuring technology, in which systems a sensor head is moved across a measuring gauge (“incremental track”) that is periodically magnetized with changing polarity. Ideally, a sine-shaped and cosine-shaped sensor signal is generated in the sensor head or magnetic field sensor by the movement (“incremental sensor”). Depending on the magnetic measuring principle, the signal course repeats itself in the sensor head with each pole of the measuring gauge (e.g. in GMR or AMR sensors which measure the square of the magnetic flow density B2) or with each pole pair of the measuring gauge (e.g. in Hall effect sensors which provide an output voltage that is proportional to the product of the magnetic field strength and current). The determination of the position on said incremental track takes place in a conventional manner, e.g. by means of trigonometric functions.
Absolute Measuring Systems with Several Tracks
There are several tracks on said measuring gauge. In addition to said incremental track of said incremental sensor, in an absolute measuring system, said sensor head contains at least one absolute sensor in order to sense one or more absolute tracks. Both the incremental sensor and the absolute sensor are thereby usually arranged in a communal sensor head housing. The incremental position specifies or feeds back the position of the sensor or sensor head within a pole or pole pair (depending on the physical measuring principle). The absolute position can be calculated using the following algorithm:position(absolute)=position(in pole)+pole number*pole width.
In order to determine the pole number, a digital pseudo-random-coded (=PRC) track having logical values 0 and 1 corresponding to the N and S poles is usually used.
Also using or evaluating pole pairs in the form of a so-called “inverted code word” in addition to said individual poles of a measuring gauge originates from WO 01/09568 A1. In a magnetic measuring system of the N pole or the S pole, said coding of logical values is thus not carried out by one individual pole, but by at least two poles respectively summarized in said absolute track in the measuring direction. By doing so, such a pole pair has double the width of the incremental track, whereby a logical value is formed by the pole sequence N-S or the amended pole sequence S-N. The advantage of this arrangement of pole pairs is that, when a logical value 0 turns into 1 or a logical value 1 into 0, two adjacent, concordant poles always emerge. These concordant poles with doubled pole width can be better sensed than poles having only a single pole width because of the doubling of the field strength. Thus the air gap in particular in such a length measuring system can be increased.